1. Field of the Invention
The present invention relates generally to methods for recording and/or reproducing digital data and, more particularly, is directed to a method for recording digital data on a recording medium in high packing density and/or reproducing the recorded digital data from the recording medium.
2. Description of the Prior Art
When a digital signal is recorded on a magnetic recording medium, it is proposed that a binary digital signal of "1" or "0" is made corresponding to a magnetization inverted layer according to a nonreturn to zero inverted (NRZI) method or that a digital signal is made corresponding to different polarities according to a nonreturn to zero (NRZ) method.
FIG. 1 schematically illustrates the magnetization condition and the corresponding codes according to the prior-art recording method. It is clear from FIG. 1 that the binary digital signal of "1" or "0" is made corresponding to the magnetization inverted layer according to the NRZI method and that the digital signal is made corresponding to the different polarities according to the NRZ method.
In these known methods, if the signal, which repeats "1" and "0", is frequently recorded, then the level of the reproduced output is lowered by a gap loss caused by the gap length of the reproducing head, a spacing loss caused by the distance between the recording medium and the magnetic head, and so on. In the digital recording and/or reproducing apparatus which is now commercially available on the market, the packing density value is determined by the output carrier-to-noise ("C/N") ratio of the maximum repetitive frequency of a pattern to be recorded.
In the recording method according to the prior art, a region having a magnetization vector in a certain direction is always accompanied with the adjacent region having a magnetization of opposite polarity. If the region of the same length is repeated, then a signal of a single cycle is recorded. FIG. 2 shows a relationship between a gap length and a recorded or reproduced output where the recorded or reproduced output is plotted while changing the cycle. In FIG. 2, the gap length (g) per wavelength (.lambda.) of the repeated signal is indicated on the abscissa and the recorded or reproduced output is indicated on the ordinate. It is to be understood from FIG. 2 that the output level is rapidly lowered when the wavelength (.lambda.) of the cycle signal approaches the gap. This determines the limit of the packing density.
Further, if the packing density is increased in the binary modulation code, the maximum repetitive frequency after being modulated is increased, and the deterioration of signal arises as shown in FIG. 3, thus frequently causing mis-detection. Alternatively, non-linear intersymbol interference, called a peak-shift, occurs due to influences other than a head magnetic field, such as a self-demagnetizing force, a mutual action of magnetic particle itself, and so forth. The interference acts to determine a residual magnetization so that recorded data can not be detected.
A loss in the recording mode is, for example, recording demagnetization, separation loss caused by the fact that the tape is distant apart from the head, thickness loss caused by the tape thickness, self-demagnetization loss occurring in the tape, and the like, whereas a loss in the reproducing mode is a gap loss, a separation loss, an azimuth loss and the like.
As described above, it was intended to increase the packing density by the use of the binary modulation code. However, various losses occur in the recording and reproducing modes so that the increase of the packing density depends on the enhancement of efficiency of the magnetic medium and the magnetic head. Therefore, it is difficult to increase the packing density satisfactorily.